As the computing power of modern devices grows, more and more data is generated and must be stored. Some data storage systems include both a primary data storage system and a backup data storage system, to reduce the overall chance of data loss. Usually the primary storage is backed up multiple times and therefore the backup storage capacity has to be greater or equal to the primary storage capacity. This can be considered an inefficient use of resources. One technique that has been used to reduce the total data storage capacity necessary for a backup storage system is data deduplication. Data deduplicating systems recognize repeated segments of data and store only a single copy of each segment, greatly reducing storage when similar files are stored. In some circumstances, it is advantageous to determine the required storage capacity for a deduplicating backup system, or whether a deduplicating backup will fit in a given storage device, before the backup is created.